This study compares the effects of Enhanced Milieu Teaching (EMT), a naturalistic conversation-based early language intervention, when implemented by parents and by therapists. Seventy preschool children with mental retardation in the early stages of language learning and their parents will participate in a randomized experiment to determine the immediate, generalized and long term effects of two protocols for providing Enhanced Milieu Teaching on children's lexical, syntactic and conversation performance. Children will be assigned to one of the two treatment conditions, parent-implemented EMT or therapist-implemented EMT. Children in both conditions will receive 36 sessions of EMT. Measures of child language use (IPSYN, MLU, diversity of word roots), target semantic/syntactic language skills, and performance on standardized assessments of language will be collected before the intervention and at four points spanning 12 months after the intervention. In addition, assessments of child generalization to the home and to an unfamiliar partner will be assessed at the pre and post periods. We hypothesize that the two protocols for intervention will have similar positive effects during training and that children receiving the parent- implemented intervention will show better generalization across partners and better language performance one year after the intervention.